This invention relates to an a.c. coupled high gain multistage operational amplifier.
Switched capacitance operational transconductance amplifiers commonly use two stages: a telescopic amplifier with a preamplifier to increase gain without degrading stability and to increase bandwidth. T. B. Cho and P. Gray, xe2x80x9cA 10b, 20M sample/s, 35 mW Pipline A/D converterxe2x80x9d, IEEE JSSC, Mar. 1995, page 166. K. Vleugels, S. Rabii, and B. Woday, xe2x80x9cA 2.5 MHz Broadband Multi-Bit xcexa3xcex94 Modulator with 95 dB Dynamic Rangexe2x80x9d, Proc. ISSCC, 2001, page 50. However, this places tight voltage headroom restrictions on the preamplifier circuit which is constrained by the bias of the input circuits of the telescopic amplifier. This constraint can be reduced somewhat if NMOS and PMOS devices are used but PMOS devices tend to reduce bandwidth and gain which is undesirable.
It is therefore an object of this invention to provide an improved a.c. coupled high gain multistage operational amplifier.
It is a further object of this invention to provide an improved a.c. coupled high gain multistage operational amplifier which decouples the stages so that the bias voltages in each stage can be independent of each other.
It is a further object of this invention to provide an improved a.c. coupled high gain multistage operational amplifier which decouples the stages so that the device areas at the input of each stage can be independent of each other.
It is a further object of this invention to provide an improved a.c. coupled high gain, high bandwidth, multistage operational amplifier which is advantageous for low voltage designs.
It is a further object of this invention to provide an improved a.c. coupled high gain multistage operational amplifier which cancels the offset of the first stage.
The invention results from the realization that an improved a.c. coupled high gain, high bandwidth, multistage operational amplifier in which the bias voltages of the input devices are independent of each other can be achieved by employing an a.c. coupling capacitance between the stages and charging that capacitance during the track phase when the operational amplifier is not operating and using the charge on the coupling capacitance to bias the next stage during the hold phase without constraining the bias on the input of the previous stage.
This invention features an a.c. coupled multistage high gain operational amplifier including at least two gain stages, each having an input and an output. There is an a.c. coupling level shifting capacitance interconnecting the output of the first stage to the input of the second stage and a charging circuit interconnecting with the a.c. coupling level shifting capacitance and the input of the second stage to charge the a.c. coupling level shifting capacitance in a track phase and to connect the a.c. coupling capacitance to the input of the second stage during a hold phase for dissociating the bias voltages of the stages.
In a preferred embodiment the charging circuit may include a common mode switching circuit interconnected with the a.c. coupling level shifting capacitance and the input of the second stage to charge the a.c. coupling level shifting capacitance in the track phase and to connect the a.c. coupling level shifting capacitance to the input of the second stage during the hold phase. There may be a common mode bias circuit for charging the a.c. coupling level shifting capacitance during the track phase. The stages may be implemented in NMOS. There may be a parasitic capacitance associated with the input of the second stage and the a.c. coupling level shifting capacitance may be larger than the parasitic capacitance. The second stage may include a telescopic amplifier. The telescopic amplifier may include a tail current source. The a.c. coupling level shifting capacitance may effect the cancellation of the offset of the first stage.